1. Field of the Invention
The present invention relates to an image forming apparatus, such as, a copying machine, a printer, a facsimile machine, a multifunctional image forming apparatus, or other similar image forming apparatuses, that forms a color image in a color mode and forms a black image in a monochrome mode.
2. Discussion of the Background
In a color image forming apparatus, such as, a copying machine, a printer, a facsimile machine, a multifunctional image forming apparatus, or other similar image forming apparatuses, both a mono-color (i.e., black) image and a multi-color image can be formed. In this type of image forming apparatus, at least one gear (hereafter may be simply referred to as a “color gear”) for driving and rotating an image carrier that carries a color toner image (hereafter may be simply referred to as a “color image carrier”) and a gear (hereafter may be simply referred to as a “black gear”) for driving and rotating an image carrier that carries a black toner image (hereafter may be simply referred to as a “black image carrier”) start rotating and stop in accordance with the start and stop of an image forming operation. In this condition, when the color gear and the black gear start rotating and stop, these gears and gears meshed with the color gear and the black gear are under heavy load conditions. Therefore, if the color gear and the black gear constantly stop at the same positions, each of the same positions of the color gear and the black gear is repeatedly under a heavy load, thereby causing the color gear and black gear to be damaged locally. As a result, the useful life of the color gear and black gear is reduced.
If the color gear and the black gear stop at positions different from their rotation start-positions, respectively, the useful life of the color gear and black gear can be prevented from reducing. However, the following problem may occur with this construction.
Generally, color gears and a black gear are arranged with predetermined phase relations kept therebetween to prevent the deviation of the position of color toner images transferred onto a transfer material. By keeping the phase relations between the color gears and the black gear, the occurrence of the deviation of the position of color toner images is effectively controlled. For example, published Japanese patent application No. 2000-187428 describes this technique. However, in a monochrome mode in which color gears and a color image carrier are halted and a black toner image is formed on a black image carrier while driving the black image carrier to rotate by the black gear, if the black gear is stopped at a position different from its rotation start-position, the predetermined phase relations between the black gear and the color gears are changed, thereby causing the occurrence of the deviation of the position of color toner images formed by subsequent image forming operations.
Therefore, it is desirable to provide an image forming apparatus that prevents local damage of color gears and a black gear, and that effectively controls the deviation of the position of color images.